There are various types of conventional apparatus for measuring the ignition timing advance angle of an internal combustion engine. In such an apparatus a signal indicative of the ignition timing advance angle is produced wherein said signal may be utilized for displaying the advance angle or giving the information of the ignition timing to a computer circuit which generates, for instance, a control signal with which the air/fuel ratio of the air/fuel mixture to be supplied to the engine is controlled. In one such type of apparatus a circular disk or body is fixedly connected to the crankshaft of an internal combustion engine so that the disk rotates in synchronization with the crank shaft. The circular disk has a plurality of gear teeth which have an equal circular pitch, along the peripheral surface of the disk. At least one marker, such as a projection, which is utilized for detecting the reference point or position of the circular disk, is mounted on the circular disk at a different position with respect to the gear teeth. Two pick-ups, which are of the electro-magnetic type, are respectively disposed on a fixed stationary member and are respectively arranged adjacent the peripheral surface and the circular track of the marker. These two pick-ups respectively emit a train of output signals when the circular disk rotates since the magnetic resistance between a first pick-up and the gear teeth and the magnetic resistance between the second pick-up and the marker vary as the disk rotates. A first signal produced by the first pick-up provides calibration pulses via which angular measurement can be made while a second signal produced by the second pick-up is used to indicate a reference time or position which corresponds to the reference position from which an angle can be measured using the calibration pulses.
In case of utilizing such an apparatus, there are two methods for increasing the accuracy of the measurement of the advance angle. The first method is to increase the number of the teeth, i.e. the calibrating members, and the second method is to employ more than two pick-ups which are aligned in a direction of the rotation of the circular disk, for producing, via suitable circuitry, a signal indicative of the phase difference between the output signals thereof. In the former case precision machining for producing the large number of gear teeth is required and thus the first method is not suited to mass-production. In the latter case, at least three pick-ups are necessary, viz. two for the teeth and one for the marker. The increase of the number of pick-ups causes a cost increase since each pick-up is relatively expensive. Moreover, if more than two pick-ups are disposed adjacent the peripheral surface of the disk, it is necessary to arrange these two-pick-ups within a circular pitch of the teeth. Further, an adjustment of the space between the pick-ups and the peripheral surface of the disk is troublesome and time consuming.
In another type of an apparatus for measuring the advance angle, only one pick up is employed for sensing both of the gear teeth and the marker. In this type of apparatus, the marker is disposed between two specific teeth, so that the pick-up which is arranged adjacent the peripheral surface of the circular disk, produces a train of pulse signals as the disk rotates. In order to distinguish a signal indicative of the reference position or angle from the other calibration pulse signals, the circular pitch between the two specific teeth is different from those of the other teeth which are equidistantly spaced or angularly displaced whereby the phases of the two signals are different from each other. Consequently, the output signal of the pick-up will be divided, by means of the phase difference, into two signals which respectively indicate the reference position and the calibration signals. In such an apparatus, a method of increasing the number of gear teeth is usually adopted for increasing the accuracy of measurement of a rotational angle since an increase in the number of pick-ups is contradictory to the purpose of using a single pick-up.
Other arrangements for measuring a rotational angle are possible. For instance, a combination of a circular disk which has a plurality of slits for providing calibration signals, a slit for providing a reference position signal, a lamp and photo-sensitive cell is known. Further, a combination of a circular disk the peripheral surface of which is magnetized to provide a plurality of N and S poles and a magneto-sensitive detector, which will be referred to as a pick up, disposed adjacent to the peripheral surface of the disk is known. However, no matter which of these known devices is utilized, the drawbacks set forth hereinabove are not overcome.